The present invention relates to gas turbine engines, particularly to a swirled diffusion dump combustor, and more particularly to a fuel and gas premixer used with a swirled diffusion dump combustor for the type of gas turbines which may be used in power plant applications.
Industrial gas turbine engines have increasingly stringent emission requirements. In order to provide a marketable power generation product, an engine producing the lowest possible emissions is crucial. Emissions of nitrogen oxides (NOx) and carbon monoxide (CO) must be minimized over specified engine operating ranges. To achieve this low level of emissions the combustion system requires the complete burning of fuel and air at law temperatures
Combustors that achieve low NOx emissions without water injection are known as dry-low emissions (DLE) and offer the prospect of clean emissions combined with high engine efficiency. This technology relies on a high air content in the fuel/air mixture. Therefore the current technology for achieving low NOx emissions may require a fuel/air premixer.
In a DLE system, fuel and air are leave-premixed prior to injection into the combustor. No diluent additions, such as water injection are needed to achieve significantly low combustion temperatures, which minimize the amount of NOx formation. However, two problems have been observed. The first is combustion instability and noise or unstable engine operability and the second relates to CO emissions and decreasing combustion efficiency. The stability of combustion rapidly decreases under lean conditions and the combustor may be operating close to its blow-out limit because of the exponential temperature dependence of the chemical reactions. This can also lead to combustion instabilities which change the dynamic behaviour of the combustion process, and endanger the mechanical integrity of the entire gas turbine engine. This is because several constraints are imposed on the homogeneity of the fuel/air mixture since leaner than average pockets of mixture may lead to stability problems, and richer than average pockets will lead to unacceptably high NOx emissions. At the same time, a substantial increase in CO and unburned hydrocarbon (UHC) emissions as a tracer for combustion efficiency is observed, which is due to the exponential decrease in chemical reaction kinetics at leaner mixtures, for a given combustor.
It has been found that a key requirement for a successful DLE combustion system is the reaction of a perfectly mixed fuel and air mixture that has a variation not greater than +/xe2x88x923% in fuel/air ratio at the inlet to the combustor. The flow field generated in the combustor must be stable to ensure complete burning of the fuel and air, while minimizing combustion noise.
Other problems relating to a combustion system in which fuel and air are premixed prior to injection into the combustor are auto-ignition and flame flashback. Premixers used for low emission combustion systems must overcome those problems as well. Efforts have been made to develop improved low emission combustion systems, particularly with fuel/air premixers, examples of which are described in U.S. patent application Ser. No. 09/742,009, entitled DIFFUSION MIXER filed on Dec. 22, 2000 and in U.S. patent application Ser. No. 09/840,991, entitled DIFFUSION COMBUSTOR, filed on Apr. 25, 2001, both assigned to the assignee of this patent application. Nevertheless, there is still a need for improved low emission combustion systems and particularly for improved premixers for such combustion systems,
It is an object of the present invention to provide a fuel and air mixer which is capable of providing a better fuel/air mixture for a low emission combustor.
It is another object of the present invention to provide a single fuel and air mixer capable of staging the fuel/air mixture supply to meet different requirements of engine operating conditions.
It is a further object of the present invention to provide a swirled diffusion dump combustor used for gas turbine engines to achieve low NOx and CO emissions from base load to part load engine operating conditions.
In accordance with one aspect of the present invention, there is a mixer provided for a gas turbine combustor. The mixer comprises an annular chamber having an upstream end and a downstream end, and a manifold ring closing the upstream end of the annular chamber. The annular chamber includes an annular inner wall and an annular outer wall to define the chamber therebetween, the annular inner wall extending downstream-wise, radially and outwardly and the annular outer wall extending downstream-wise radially and inwardly. The manifold ring includes a fuel passage in fluid communication with the annular chamber for feeding fuel into the annular chamber, and a plurality swirled air passages to provide swirled compressor air flows into the annular chamber. The swirled air flows mix with fuel from the fuel passages, thereby producing a fuel/air mixture in the annular chamber. A downstream end of the annular chamber is adapted to be connected to the combustor in fluid communication therewith for dumping the fuel/air mixture into the combustor for combustion.
The fuel passage is preferably farmed by a fuel ring coaxial with the annular chamber. The fuel ring preferably includes annular inner and outer walls extending from the manifold ring downstream-wise to define an annular fuel passage with a plurality of holes in a downstream end of the fuel ring. The holes are located in a circumferentially spaced apart relationship. The fuel ring according to one embodiment of the present invention includes two radially positioned buffer plates circumferentially spaced apart from each other to divide the annular passage into two passage sections, permitting fuel delivery through either passage sections or through both sections simultaneously so that local fuel and air mixing ratios can he adjusted without changing the overall fuel and air flow mass.
The swirled air passages preferably include first and second groups of air passages extending through the manifold ring and distributed in a circumferentially spaced apart relationship along respective first and second circular lines coaxial with the first fuel ring. The first circular line has a diameter smaller than the diameter of the fuel ring, and the second circular line has a diameter greater than the diameter of the fuel ring.
The air passages in the respective first and second groups according to one embodiment of the present-invention are tangentially inclined in one rotational direction, either clockwise or counter-clockwise, to produce a spiral air flow is the annular chamber, which results in a relatively stable flame in the combustor. In another embodiment of the present invention, the air passages in one of the first and second groups are tangentially inclined in a clockwise direction while the air passages of the other group are inclined in a counter-clockwise direction to produce air turbulence in the annular chamber of the mixer, which results in a better mixing of fuel and air.
It is preferable to provide a downstream annular passage defined between cylindrical inner and outer walls extending downstream-wise from the downstream end of the annular chamber. The downstream annular passage serves as a region of diffusive mixing and is adapted to be connected to the combustor in fluid communication for dumping the fuel/air mixture from the annular chamber into the combustor for combustion.
In accordance with another aspect of the present invention, a gas turbine combustor is provided. The combustor comprises a cylindrical combustor can for receiving a fuel/air mixture to produce combustion products. The combustor can has a central axis and includes an annular side wall and opposed upstream and downstream ends. At least one igniter is positioned inside the combustor can and is attached to the combustor can. The mixer according to the present invention is attached to the upstream end of a combustor can, in a coaxial relationship. It is preferable that an end plate be attached to an end periphery of the inner wall of the downstream annular passage of the mixer, thereby forming a central portion of an upstream end wall of the combustor can such that an annular opening at the upstream end is formed around the center portion of the upstream end wall thereof. The annular opening does not interfere with the mixture flow passing therethrough so that the dynamic features of the fuel/air mixture obtained from the mixing process in the mixer will not be affected when the fuel/air mixture is dumped into the combustor can for combustion.
The central aperture of the fuel ring which is in fluid communication with a central passage defined within the annular inner wall of the annular chamber, preferably receives a pilot fuel line extending therethrough and connected to the central portion of the upstream end wall of the combustor can for delivering fuel into the combustor can. A pilot flame provides a stabilizing diffusion flame at part load conditions. The central portion of the upstream end wall preferably includes a plurality of holes for admission of air flows from the central aperture and the central, passage to cool the upstream end wall of the combustor can. The mixer according to the present invention is able to provide a fuel/air mixture with a mixing ratio variation of less than +/xe2x88x923% at the inlet to the combustor. Therefore the swirled diffusion dump combustor according to the present invention advantageously achieves low emissions with NOx lower than 10 ppm and CO lower than 20 ppm from base load to part load conditions. Furthermore, the structures of the mixer of the present invention effectively prevents auto-ignition and flame flashback. The burning fuel/air mixture in the primary combustion zone of the combustor is stabilized by the swirl generated in the annular chamber of the mixer and by the pressure gradient induced circulation toward the upstream end wall of the combustor can.
Other advantages and features of the present invention will be better understood with reference to preferred embodiments of the present invention described hereinafter.